Method of plating and article produced thereby



Patented Nov. 17, 1936 PATENT OFFICE M THOD OF PLATING AND ARTICLEPRODUCED THEREBY Edwin M. Baker, Ann Arbor, Mich., and Leslie 0.Borcliert, Chicago, 111., assignors to General Spring BumperCorporation, Detroit, Mich., a. corporation of Michigan No Drawing.Application December 13, 1933,

Serial No. 702,196

8 Claims. (01. 204-113) This invention relates to a method of platingand to an article produced thereby, having more particularly referenceto the protection of ferrous metals against corrosion by theelectrodeposition on the ferrousmetal base of com-' posite metalliclayers.

It has heretofore been common practice, in the protection of ferrousmetals and their alloys against corrosion, to plate over the ferrousmetal base with a layer of metal that is more resistant to corrosionthan iron, such as nickel or chromium, or nickel and chromium. It hasalso been customary to apply a coating of copper first directly to theferrous metal base and then coat overthe copper with nickel, or withnickel and chromium. It is well known, however, that deposits of metal,such as nickel and copper protect the underlying ferrous metal base, bywhich is meant iron and its alloys, such as steeL-from corrosion onlyinsofar as the deposits are continuous or non-porous. It has been shownby many researches, starting in particular with that published by E. M.Baker in the Journal of the Society of Automotive Engineers, February,i924, Rust Resistance of Nickel-Plated Steel, that it is commerciallyimpossible to produce an absolutely impervious or non-porous deposit,although'the porosity may be decreased progressively by increasing thethickness of the deposit. On the other hand, an increase in thickness ofthe deposit constitutes an increasing expense, because of the longertime which the article must remain in the plating solution, because ofthe greater consumption of expensive anodes, and because of the cost forpower. Fur thermore, thick deposits are more difllcult and moreexpensive to buff than are thinner deposits.

Previous workers in the field have proposed to obtain betterresistanceto rusting by the use of a layer of electrodeposited zinc orcadmium applied directly to the ferrous metal base, on the theory thatsince zinc and cadmium are electropositivein the electromotive forceseries toward iron, the iron or iron ,alloy base will be protected solong as any zinc or cadmium remains in contact therewith. Althoughrusting is, in fact, retarded by the use of undercoats of zinc orcadmium, another and very serious difliculty is experienced. If there isa holeor pore in the deposit of nickel, and/or chromium, overlying thezinc or cadmium, rapid corrosion of the zinc or cadmium occurs with theformation of a white corrosionproduct which spreads over the finishedsurface and greatly mars its appearance.

Furthermore, the corrosion product, having a greater volume than thezinc or cadmium from which it was formed, causes the overlying plate torise or form a blister of non-adherent metal at the point where poresand corrosion occur. For this reason, we have not found it satisfactoryor feasible to use a. deposit of zinc or cadmium to improve rustresistance.

It has also been suggested that the underplate of copper might beomitted entirely and the nickel plated directly on the ferrous metalbase. For steel plated by research methods, this procedure givesexcellent results, as it is attested by recent publications from the U.S. Bureau of Standards. In commercial work, however, the use of a singledeposit has one very serious objection. Work, after being plated, isalmost universally buffed, as by means of wheels made up of disks ofcotton cloth and rotated at high speeds. A compound of grease andabrasive is applied to the surface of the wheel. The part to be buffedis held with considerable pressure against the bufllng wheel so coated.Careless and inexperienced operators will not infrequently remove all ofthe plate from very small areas, leaving the ferrous metal base exposed.When this happens, there is, of course, no protection offered at thepoint where the nickel has been buffed away, but this is not so seriousbecause the completely buffed through parts will be rejected in theplant by the nickel bufling inspector and will be replated. However,when some articles are buffed through clear to the steel, it is evidentthat many more may be buffed to a point where the nickel is notcompletely removed, but where it has been reduced to an extremely thinlayer. This condition cannot be ascertained by visual inspection and incommercial work the use of a. single deposit of nickel presents thisgreat danger of unknowingly shipping plated partswhich will have toopoor resistance to rusting and which will, therefore, constitute aserious risk of loss of reputation for quality of products.

The use of an intermediate plating of copper,

obviates this danger of bufl'lng through the plated coating. Forinstance, if the composite deposit consists of a layer of nickel overthe ferrous metal base, then a layer of copper and then another layer ofnickel, if the buffer operator bufis through at any point to a depthwhere the deposit of copper will be reached, this condition is readilyfound onvisual inspection because of the difference in colors of nickeland copper. If the article is not buffed through to the copper, ittherefore follows that it is coated with the full first deposit ofnickel, the full first depositof copper, and at least a portion of thefinal deposit of nickel.

It is an object of our invention in particular to overcome thedisadvantages inherent in all of the above procedures and to utilizetheir good points in a process of plating. According to our invention,we have substituted 9. layer of electrodeposited brass for the layer ofcopper in a composite coating such as above referred to. The color ofbrass is sufiiciently different from that of nickel so that theinspection feature is provided.

Also, according to our invention, we derive the benefit in increasedcorrosion resistant properties that result from the use of anunderplating that has a potential which is higher in the E. M. F. seriesthan that of iron or is only slightly lower than that of iron, since thepresence of zinc in the brass plating changes the E. M. F. from that ofcopper, which is below iron, to one that may be practically the same asthat of iron or above. We may vary over a considerable range thecomposition of the brass deposit, but if the reentage of copper is veryhigh and the perce tage of zinc low, the brass deposit will have an E.M. F. which will lie close to that of copper and will not well serve ourpurpose, because if there is a pore in the deposit, it will, likecopper, tend to promote corrosion of the underlying steel but to a lessextent than pure copper. As the percentage of copper in the brass isdecreased and the percentage of zinc increased, the potential of thebrass will become more like the potential of nickel. By so increasingthe percentage of zinc, the potential can be brought down to that ofnickel. may be made intermediate between the potential of nickel andthat of iron, or, on further increase of zinc, may be brought to thepotential of iron or may be made intermediate between that of iron andthat of zinc.

While, in general, the advantages of our invention can be obtained to adegree with brass deposits of widely varying compositions, best resultsare obtained when the composition of brass is selected so as to give anelectrolytic potential between that of nickel and that of iron, in thesolution, which may be in contact with the article. By the term,solution is meant any film of moisture that may be in contact with theplated article. For instance, in the case of automotive bumpers, a filmof moisture will form on the bumper on rainy or moist days and maycontain dissolved acid gases from the air or dissolved salts which havebeen splashed up from the street.

In some cases, it may be desirable to make the composition of the brasssuch that the potential will be close to iron but between iron and zinc.In such case, the galvanic action will be such that the brass depositwill protect the underlying ferrous metal base from corrosion evenalthough.

there be slight porosity through all of the plated metals. However, thepercentage of zinc in the brass should not be increased to a point wherecorrosion will cause blistering of the deposit and the formation ofundesirable white iucrustations on the outside surface of the plate.

It is, therefore, an important object of this invention to provide acommercially workable proces of plating iron, steel or other ferrousmetal alloy with metals having an outer surface of nickel, and/orchromium, and a superior resistance to rusting or corrosion when theplated article is exposed to atmospheric conditions or to acceleratedcorrosion tests.

Other and further important objects of this invention will becomeapparent from the following description and appended claims.

Since our invention is especially well adapted for use in themanufacture of an automotive vehicle bumper and the like, it will bedescribed in connection therewith, although it should be understood thatthe general principles of our invention are of wider application. In thepractice of our process in connection with the nickel plating orchromium plating of a steel bar or plate, such as is employed in themanufacture of a vehicle bumper, a bar is first polished on suchsurfaces as require a bright or image reflecting surface on the finishedproduct. The bar is then thoroughly cleaned in any suitable manneremploying for this purpose means and solutions commonly used in thepractice of the plating art. For example, the bumper bar may be firstimmersed in a hot or boiling alkaline solution, such as a solutioncontaining 2 to 4 ounces per gallon of trisodium phosphate or othermixtures of alkaline salts. Electric current may or may not be used toaid the cleaning and in such case, the bumper bar to be cleaned may bemade either the anode or the cathode.

After this cleaning operation, the bumper bar may be rinsed in water andmay or may not be scrubbed with fine pumice stone and again rinsed. Itis good practice to then dip the bumper bar for perhaps 30 seconds in a10% solution of muriatic acidand to again rinse in water. The bumper baris then ready for plating. Up to this point, the operations describedare those common to the art.

After completion of the foregoing cleaning operations, it is customaryto plate the bumper bar with three successive coatings of metal asfollows: A first or flash coating of nickel, which, for example, maybe0.0003 inches thick; a second coating consisting of copper, which may beabout the same thickness as the first coating of nickel; a third coatingconsisting of nickel, which may, for example, be twice the thickness ofthe flash coating of nickel. Any suitable composition of nickel andcopper plating baths, respectively, may be used for the nickel andcopperplating. After the final nickel plating, the bumper bar is buffedon buffing wheels using an abrasive compound. If the article is to befinished with a top surface of nickel, the operation is then complete.If, however, the article is to be'chromium plated, it is subsequentlyplated with chromium, using procedures commonly employed in the art forthat purpose. v

The improvement in the art that constitutes this invention is thesubstitution of a suitable deposit of brass for the deposit of copper.We have found such a deposit of brass to have many advantages over acopper deposit and in addition to have the same advantage of permittingready visual inspection of the buffed article that copper has. The mainadvantage, as previously pointed out, in the use of a deposition ofbrass, rather than copper, is that the composition of the brass may beso selected that its electromotive force lies above that of nickel andpreferably between nickel and iron, so that even if there are pores inthe nickel layer, the nickel will be protected against corrosion.Furthermore, by using a deposit of brass rather than copper, the ferrousmetal base will be protected to a much greater degree, in the event ofpores being present in the coated layers, owing to the fact that theelectromotive force of the brass deposit is so much closer to that ofthe ferrous metal base than is that of copper.

The composition of the brass may be varied through a comparatively widerange, with the copper between 45 and 65% and the zinc between 55 andPreferably, we employ a composition of brass containing about 60% copperand zinc. Cadmium may be substituted in part or in whole for the zinc inthe brass deposit to obtain the advantages of our invention, but weprefer to use zinc because it is a simpler matter to deposit azinc-copper brass than it is to deposit a cadmium-copper brass.

Our invention contemplates many variations in the number and sequence ofthe coatings to be applied to the ferrous metal base. These may beindicated as follows: i

1. Nickel on brass on nickel on ferrous metal.

2. Nickel on brass on brass on ferrous metal.

3. Nickel on brass on nickel on brass on ferrous metal. h 4. Nickel onbrass on nickel on copper on ferrous metal. 5. Nickel on brass on copperon ferrous alloy. The presence of a copper layer would harm rather-thanhelp the development of rust resist:-

ance, but its presence would not make the layer of brass ineflfeclve infulfilling its ownfunction.

Although, as is obvious from the abovdescription, any suitablecomposition and operating conditions may be used for any and all of theplating operations, for purposes of illustration, we nevertheless givethe following compositions of bath and operating conditions which havebeen found suitable for the practice of this invention.

For the nickel plating bath, we prepare a solution in which thefollowing ingredients are employed in about the proportions stated foreach gallon of solution:

32 ounces of nickel sulfate (NiSO4.7H2O) 8.0 ounces of nickel chloride(NiChfiHaO) 4.0 ounces of nickel boric acid (HaBOs) The pH of the bathis preferably adjusted to between 2 and 3. The proportions above statedmay be varied somewhat with comparable results. The solution is heated(as by means of a steam coil) to a temperature of from 100 to 125 F.,the best results 'having been obtained by employing a temperature ofabout 115 F. For the anodes, substantially pure nickel (at least 99%pure) is employed. -With a bath constituted and heated in the mannerdescribed, and employing such pure nickel anodes, the work is platedwith a current density of approximately 40 amperes per square foot ofmetal treated, but this may be varied somewhat.

For the brass plating operation, a bath is used containing the followingingredients, preferably in the proportions stated:

Ounces per gallon Copper cyanide (CIICN) g 4.8 Zinc cyanide (ZnCN-z) 2Sodium cyanide (NaCN) I 8 to 12 Sodium carbonate (Na-.-CO:.10H:O)---- 3Sodium thiosulfate (NazSaOa) 0.1 to 0.2 Potassium hydroxide (KOH) 0.05Ammonia solution (28% N'Ha) fluid The amount of ammonia added isdetermined by the appearance of the deposit and enough ammonia is addedto give a clear, bright deposit. Small amounts of cadmium cyanide orother addition agents may be used, if desired, to increase thebrightness of the brass deposit.

The solution of the brass plating bath is heated by means of asteam coilto a temperature from 95 to 110 F., preferably to about 105 E,

although this temperature is not believed to be critical. Anodes ofbrasapreferably containing over 99% of copper plus zinc, and preferablycontaining zinc and copper in the proportion of parts of copper to 40parts of zinc, are used.

- Since the composition of the deposit of brass will, in the long run,tend to become the same as the composition of the anodes, it is obviousthat the relative proportions of the copper and zinc in the anodes canbe varied over rather wide limits and still obtain satisfactory results.

If there were no loss of zinc or cadmium cyanides from the solution, andif no zinc or cadmium cyanides were added to the solution, the

' composition of the deposit would eventually becopper cyanides to thesolution. If these are not added in the some-proportion as they wouldexist in the solution if there were no loss of solution, then thecomposition of the deposit will correspondingly be different from thecomposition of the anodes. It may not be necessary to add copper andzinc cyanides to make up for drag-out loss, because the anode currentefficiency will in general be higher than the cathode current emciency,in which case only sodium cyanide, sodium thiosulfate and ammonia needbe added for the continued operation of the solution.

The current density used may be varied considerably, but preferably acurrent density of 10 to 30 amperes per square foot of surface to beplated is employed. The composition of the anodes and of the bath are soselected and controlled as to give a deposit of brass ranging incomposition from copper and 35% zinc to 50% copper and 50% zinc. A moresatisfactory range is from copper and 30% zinc to 50% copper and 50%zinc, with the preferred proportions of 60% copper and 40% zinc.

A brass deposit within the foregoing ranges of proportions of copper andzinc will have an E. M. F. between that of nickel and zinc, and if the60% copper-40% zinc composition is employed, its-E. M. 'I". will liebetween nickel and iron.

Since there is some discrepancy between different tables showing theelectromotive force series of the metals, the following is given as theapproximate single potential of each respective metal toward a solutionof normal metal ion concentration or activity, based on the normalhydrogen electrodes:

Volts Zinc 0.76 Cadmium .40 Ir .34 Nickel .23 Copper .33

These figures are quoted from .Principles and Practice of Electroplatingand Electroforming,

by Blum and Hogaboom, published by McGraw- Hill Book Company, SecondEdition, page 407.

Where the composition of the brass deposit is close to the higher limitsfor the copper, namely, around 80% copper, the brass deposit has an E..M. F. which lies close to that of copper and will, therefore, notpossess the advantages of our invention to the maximum extent. A brassplating of this composition will, like copper, but to a lesser extent,tend to promote corrosion of the underlying ferrous metal or steel. Bestresults are obtained using the preferred composition of about 60% copperand zinc, since with that composition, the electrolytic potential of thebrass layer lies between that of nickel and that of iron. If the zinc iscarried too high, say above corrosion will cause a blistered deposit andthe formation of undesirable white incrustations on the outside of theplate.

As previously stated, the finished article may have an exterior coatingof chromium, which may be applied in a chromium plating bath of theusual composition and under the conditions now being commonly practiced.

We are aware that numerous details of the process may be varied througha wide range without departing from the principles of this invention,and we, therefore, do not purpose limiting the patent granted hereonotherwise than necessitated by the prior art.

We claim as our inventionr 1.'A method of plating ferrous metalarticles, which comprises electrodepositing directly on the ferrousmetal a layer of copper zinc alloy that is higher than nickel in the E.M. F. series and electrodepositing a layer of nickel over said copperalloy layer.

2. In a method of protecting ferrous metals against corrosion, the stepof interposing an electrodeposited layer of a brass that is higher thannickel in the E. M. F. series between an exterior plating layer ofnickel and the ferrous metal base.

3. In a method of nickel and chromium plating articles having a ferrousmetal base, the steps of first protecting the ferrous metal base byelectrodepositing directly thereon a layer of brass that is higher thannickel in the E. M. F. series and then electrodepositing nickel directlyon said brass layer.

4. As an article of manufacture, a piece of ferrous metal having acomposite coating electrodeposited thereon comprising a layer of a brassthat is higher than nickel in the E. M. F. series and a layer of nickelthereover.

5. As an article of manufacture, a piece of ferrous metal having acomposite coating electrodeposited thereon comprising a layer of a brassthat is higher than nickel in the E. M. F. series directlyelectrodeposited on said ferrous metal and a layer of nickel thereover.

6. As an article of manufacture, a piece of ferrous metal having acomposite metallic coating electrodeposited thereon comprising a layerdirectly superimposed on said ferrous metal of a copper-zinc alloy thatis higher than nickel in the E. M. F. series and a layer of nickeldirectly on said copper-zinc alloy.

7. As an article of manufacture, a piece of ferrous metal having acomposite electrodeposited metallic coating thereon comprising anintermediate layer of a brass having a composition of about copper and40% zinc and a layer of nickel over said brass layer.

8. As an article of manufacture, a piece of steel having a compositeelectrodeposited metallic coating comprising a layer of nickel over alayer of brass over a layer of nickel directly deposited on said steel,the brass having a potential between that of nickel and iron in the E.M. F. series.

EDWIN M. BAKER. LESLIE C. BORCHERT.

